HELP - Problem with my Cells (TS)

Hi all, I am a beginner in electric transport. The question is a few months ago I bought a used chinese electric scooter. Is a 5Kw, motor is in rear wheel, 72V/40Ah Thunder sky batteries (24 cells) with BMS cell by cell.

Until last week everything was perfect, the scooter have 3000kms and I can ride 70kms per charge at 85kms/h.

The problem is now range is 35kms per charge. I have tested voltage cell by cell and one of them is 0 and cell is swollen. When scooter stops voltage cell by cell is 2,6V, When scooter is full charged voltage cell by cell is 3,5/3,6 except damaged cell with 0 volts.

The question is how is this possible if I have a BMS?, maybe it does not work? How can I know if BMS is working or not?

so before putting a new pack in, it may be worth while to see what the charger does,

this will be tedious, but,

during a full charge cycle, use a multimeter to measure the voltage.

the charge profile should be constant current until voltage reaches ~87.6v.
then constant 87.6v until current falls to ~2A (using a clamp meter will make current measurement easier.
a "10A" multimeter is rated for 10A continuous, so don't hardwire it in.

aside from the heavy wires to the battery, and to the powerpoint, are there any other wires that go to the charger?

does the charger still run with those wires disconnected?

btw, replacing individual cells won't give you your range back.
you will need to replace all of them.

if you do replace the dead cell, instead of just bypassing it, discharge all the cells to ~2.6v at no load (this will be time consuming).

My experience has been that an over-discharged cell will swell too. I once was testing a retired suspect cell and got distracted, and found the cell discharged (at about 17 amps) to zero the next day. The completely discharged cell had moderately bulging sides and also the sound of liquid electrolyte sloshing about when shaken close to the ear.

When recharged, this cell contracted back to normal dimensions, the excess electrolyte was re-absorbed, and surprisingly the cell still had 80% of the specified capacity - although this was only a single cycle, and at just 17 amps.

Also, a cell voltage of just 2.6 volts under no load suggests that they have been probably drained to practically zero volts under load. Even the controllers low-voltage cutof wasn't working?

When I pushed the range of my scooter once and drained a weak cell down to my BMS's 2.1 volt cutoff (at 50-90 amps), it still bounced back to about 3.0 volts under no load.

My experience has been that an over-discharged cell will swell too. I once was testing a retired suspect cell and got distracted, and found the cell discharged (at about 17 amps) to zero the next day. The completely discharged cell had moderately bulging sides and also the sound of liquid electrolyte sloshing about when shaken close to the ear.

When recharged, this cell contracted back to normal dimensions, the excess electrolyte was re-absorbed, and surprisingly the cell still had 80% of the specified capacity - although this was only a single cycle, and at just 17 amps.

I too have made cells swell from overdischarge, though having one cell at 0v while all the rest are at 2.6v would suggest sudden capacity loss on one cell (or imbalance).

Overcharge on the charge cycle could do it,
If the bms involves a shunt, a shorted shunt could also do it (but not affect the rest of the pack)

PJD wrote:

Also, a cell voltage of just 2.6 volts under no load suggests that they have been probably drained to practically zero volts under load. Even the controllers low-voltage cutof wasn't working?

When I pushed the range of my scooter once and drained a weak cell down to my BMS's 2.1 volt cutoff (at 50-90 amps), it still bounced back to about 3.0 volts under no load.

it depends what the controllers low voltage limit was, and how graceful it is.

newer controllers don't shut down, but rather maintain the minimum voltage at the battery, effectively progressively reducing the max power until the battery is at the minimum at no load.

2.6v x 23 cells = 59.8v, or 2.5v cell average for 24 cells.

had the scooters top speed been reduced before it stopped moving all together?
did it stop moving under its own power?

on my own old emax, my acceleration current used to be 220A max (with my 400A kelly controller), I could pull every cell down to around 2v even off a full charge.
As the battery discharged, the sag didn't change, just the max current reduced.

My BMS is an early-version kit-built Gary-Goodrum BMS from the Endless Sphere forum.

The LVC uses a 2.1 volt TC54 voltage detector that switches on an optocoupler when any cell voltage goes below 2.1 volts. I have it wired with a pullup resistor to ground-out the throttle signal. The result is fairly abrupt. From initial loss of throttle to to only being able to go very slowly with the throttle barely cracked, and being unable to climb any hills, is only a km or so.

Thanks to everybody for informations and suggestions. I did the next when new TS cell arrived: I decided to remove the BMS system, I am not sure it is working rightly and I suspect it is the reasson about swell in cell for overdischarging. When I placed my new cell and I tested the cells during charging process I had to balance the new cell for three times because new cells goes to 4V and others were 3,3/3,4V.

But after that my set is now correct, after charging total voltage is near to 86,5 and avery cell is exactly 3,6V.

So, I am now without BMS but after 400kms, range is the same or slightly better. I am thinking in placed a new BMS, any suggestion about it?.